The present invention relates to electric rotary machines such as dynamoelectric machines and, more particularly, to electric rotary machines having a rotor provided with a superconducting winding, i.e. a superconducting rotor.
In general, superconducting rotors are used in so-called superconducting condition in which the field winding is kept under an atmosphere of extremely low temperature of, for example, 20.degree. K., so that the resistance in the winding may be largely reduced.
In order to protect this superconducting winding at the superconducting condition from influences of magnetic flux of the stator, and in order to keep the extremely low temperature atmosphere against the radiated heat, a cylindrical shielding body is provided around the superconducting field winding. It is strictly required to protect the superconducting winding against the alternating magnetic flux, since the superconducting condition is easily broken by an invasion by an alternating magnetic flux of several Hz from the stator side.
For this reason, the shielding body must be made of a material having a sufficiently low electric resistance, e.g. pure copper, silver or aluminum.
However, unfortunately, these conductive materials in general exhibit a small yield point and Young's modulus, so that there remains a fear that the shielding body may be plastically or permanently deformed due to a centrifugal force, when the rotor is rotated at a high speed of, for example, 3600 r.p.m. To avoid such a deformation, it has been proposed to closely fit a cylindrical reinforcing body of a material having a large Young's modulus and a high strength to the outer peripheral surface of the shielding body, so that the shielding body is capable of withstanding the centrifugal force. (For information, structures of this kind are shown in FIGS. 1 thru 3 of U.S. Pat. No. 3,679,920).
In addition to the centrifugal force, the shielding body is often subjected to an extremely large electromagnetic force, in case of a failure or accident in the dynamo itself or in the exterior power transmission system, which is superimposed to the centrifugal force. The electromagnetic force follows a sine wave curve-spreading in and out of the shielding body, so that the shielding body is subjected to a bending force to cause deformation thereof in the direction opposite to the electromagnetic force.
Therefore, when a pure conductive material is used for the shielding body, the shielding body, in the form of a cylinder, as a whole is apt to be buckled due to excessively large deformation, and a small Young's modulus of the material.
The tendency of the buckling will be reduced by positioning the externally provided reinforcing body at the inside of the shielding member, in close fitting relationship to the latter. The resistance against the centrifugal force will be also increased, when the fitting surfaces of the cylindrical reinforcing body and the shielding member are adhered to each other.
For the bending force, the stress gets larger in proportion to the distance from the neutral line of the bending, so that the maximum stress appears in the peripheral surface of the shielding body. This maximum stress becomes considerably large, when the shielding body and the cylindrical reinforcing body of different materials are superimposed or laminated, because the neutral surface of bending exists in the reinforcing body, presenting a potential for breakage of the shielding body.
The same potential would be caused by positioning a cylindrical reinforcing body on the outer peripheral surface of the shielding body with close fitting by means of adhesion of the fitting surfaces.